This invention relates to a device for amplifying and sampling multiplexed analog signals of large dynamic range, i.e. whose level may vary to a large extent.
The device according to the invention is particularly adapted to be incorporated into a seismic data acquisition chain between a multiplexer and an analog-to-digital converter.
According to a known process, the seismic signals issued from each geophone of geophone group are directly supplied to the inputs of an analog multiplexer after a preliminary amplification in a preamplifier of fixed gain. The output of the multiplexer, where a sequence of signals is available, is connected to a single amplifier made up of a chain of amplification elements arranged in series, the output of each element being connected to the input of the next element. The gain of each amplification element is fixed and preferably selected as a whole power of number two.
Since the amplitude variation from a specific sample to the following one can be very high, a selection member is used to select the number of amplification units through which the signal is to pass in order to reach an optimum amplification level, and to switch the output of the amplification element where this signal is made available to the input of an analog-to-digital converter, which is connected to a recording system. According to a first embodiment of the prior art, described for example, in French Patent No. 2.110.758, all the amplification elements have an identical gain. According to another embodiment, the gains of the amplification elements differ from one another and have respective values which are the successive powers of number two.
An example of another prior art embodiment is described in the French Patent Application No. 2.373.914 which concerns an n-stage amplifier wherein the gain of any amplifier element is equal to the square of the gain of the preceding amplifier element.
The amplifiers formed of a series of serially interconnected amplifier elements suffer from numerous disadvantages:
they are relatively slow in view of the cumulative character of the delays which affect the signals having to pass through the different amplifier elements;
the effects of the relatively slow response of each amplifier element, when operated with pulses, characterized by its slew rate, accumulated over the whole chain of serially connected elements, tends to reduce the overall speed of the amplifier response;
the time the gain selector requires to select the output of the stage where the amplitude of the amplified sample is optimum, is in proportion of the length of the amplification chain;
when it is desired to increase the gain at each stage so as to shorten the amplification chain, it becomes necessary to make use of an analog-to-digital converter capable of processing longer digital words in order to obtain the same accuracy of the mantissa values;
It must be further observed that the amplifier elements are likely to saturate when a sample of low amplitude is followed with another one of substantially higher value.
A further disadvantage of the data acquisition chains having a head multiplexer results from the fact that, in order to memorize a sample value during the time required for the gain selection, it is necessary to make use of a memory element known in the art as sample-and-hold unit, connected before the amplifier and, accordingly, receiving signals in a wide dynamic range and which must therefore be capable of memorizing signals of high level as well as those of low level.
However, it is known that the sample-and-hold units available in practice have the disadvantage of not being capable of successively memorizing two samples without interaction of one sample with the other (i.e., the crosstalk phenomenom) so that the memorized value of the amplitude of a sample is accordingly not independent from the memorized value of the amplitude of the preceding sample.
Errors due to crosstalk being applied to the input of an amplifier, are likely to be amplified with a gain factor equal to the maximum gain of the amplification chain. Moreover the sample-and-hold units generally consist of a memorizing capacitor and two intermediate amplification elements forming a buffer, which make the amplification chain longer.
Accordingly, five to six amplification elements at least are required to form the amplifier.